The invention concerns a discharge vessel for high pressure sodium vapour lamps, comprising a tubular wall made of light-transmitting material, two closing elements provided with stoppers, preferably of ceramic material, for hermetically sealing the ends of the tube by means of a bond without the use of an exhaust tube, a current lead-in wire with hermetically sealed entry into the discharge tube and an electrode joined, preferably via a stem, to the current lead-in wire and filling in the interior of the sealed tube containing a noble or inert gas and metal additive(s), preferably sodium, mercury and/or cadmium. The discharge vessel according to the invention is intended to be used as a component of high-efficiency, high-pressure sodium vapour lamps applicable in the widest variety of lighting applications, which can assure, even when employing structural materials of poorer quality are used and less careful preparation and manufacture, long service life, uniformity of parameters and reliability for such lamps.
In the manufacturing process of the tubular discharge vessels for high pressure sodium vapour lamps the two ends of the transparent or translucent tube are hermetically sealed by means of closing elements provided with stoppers. A current lead-wire is vacuum-tightly embedded in the stopper and is connected to an electrode arranged in the interior of the tube. The basic material of the tube is alumina; a part of the stopper may also be made of alumina but may additionally contain metallic parts. The components made of alumina oxide and the metallic components of the electric lead-in wires are generally bonded by vitreous enamels of high melting point, which are also suitable for producing a hermetic closure. A fill of noble gas and suitable metal additive(s), particularly sodium, mercury and/or cadmium, are passed into the interior of the tube.
When high pressure sodium vapour lamps comprising gas discharge tubes are activated by applying the required voltage to the electrode, an arc discharge occurs through the noble gas, and the supply voltage and the circuitry (in the simplest form, a choke coil) connected in series with the lamp generates a self-sustaining arc which engenders an increase of the vapour pressure of the metallic additive(s) in the discharge vessel (i.e. sodium, as well as mercury and/or cadmium) and thereby an increase of the operating voltage of the discharge. This process continues until an equilibrium sets in. In this equilibrium of the discharge, the metallic additive(s) are transformed into a liquid phase and their total vapour pressure is of the order of magnitude of 10.sup.5 Pa, depending on the composition of the melt of the metal additives in the discharge tube and on the value of the lowest surface temperature of the additives. For given dimensions and geometry of the discharge vessel, the ambient temperature, the series circuitry and the supply voltage, the electrical and optical parameters of the discharge are predominantly determined by the partial pressure(s) of the metal additive(s).
In the manufacturing process of discharge vessels for high pressure sodium vapour lamps there are two known methods for bringing the filling material into the discharge tube and for sealing the tube.
Among others, U.S. Pat. No. 3,243,635 and British Pat. No. 1,065,023 describe a method for the production of a discharge vessel, wherein an exhaust tube is used. As a first step of the process an intermediate product is created, wherein the interior of the discharge vessel and the ambient environment communicate via a thin-walled metallic tube known as the exhaust tube (usually made of niobium or a niobium alloy) connected into the stopper of the tube and having a coefficient of thermal expansion approximately matching that of alumina. The discharge vessel is evacuated through the metallic exhaust tube, thereafter the necessary filling is introduced into the interior of the discharge vessel through the same metallic exhaust tube and then the outer end of the exhaust tube is hermetically sealed. In the discharge vessel produced in this way, the tip or stump of the sealed metal exhaust tube becomes both the part for the current lead-in and the part of the discharge vessel which has the lowest temperature, i.e. it forms a so-called `cold spot` of the discharge tube. The metal additives collect here during operation of the lamp.
The manufacture of discharge vessels with exhaust tubes is relatively expensive requiring complicated special purpose machinery and the use of an exhaust tube renders the conditions for mass-production more difficult.
To simplify the manufacturing process, discharge vessels without exhaust tube have been developed, wherein the introduction of the filling and the sealing of the discharge tube are achieved by other methods. One of the ends of the discharge tube is provided with an electric current lead-in wire and is hermetically sealed by a stopper. Thereafter the discharge vessel is turned over in such a way that its closed end points downwards, the metal additives are filled into the tube via the open top end of the tube and then the closing components are placed into the top end of the tube. This is followed by distributing the basic materials of a vitreous enamel over the closing elements in a quantity and arrangement which after fusion enables the vitreous enamel to flow into any gaps that are still open. Hereafter the whole assembly, or a plurality of assemblies simultaneously is (are) placed into a suitable chamber and their upper end is heated while the lower, closed end (where, due to gravity, the metal additives accumulate) is kept at a temperature low enough for the vapour pressure of the metal additives to be negligible. The chamber is first evacuated then filled with the atmosphere of noble (inert) gas that is desired for the finished discharge vessel. Since at this stage the upper end of the discharge vessel has not yet been sealed hermetically, the pressure and composition of the gas in the inner space of the vessel will be the same as in the chamber. As the next step, the temperature is increased until the molten vitreous enamel flows into fissures or gaps and while the temperature is lowered the upper end of the tube is hermetically closed. The quantity of the gas filled into the discharge vessel is controlled by the gas pressure in the chamber.
There are several known versions of the system without exhaust tube, differing mainly in the manner of forming the current lead-in: thus e.g. according to German Pat. No. 1,639,086 the current lead-in is a niobium tube whose inner end is closed; while according to U.S. Pat. No. 3,693,007 the current lead-in is formed by a metallic layer deposited on the surface of a ceramic stopper, while according to U.S. Pat. No. 4,376,905 it is formed possibly by a plurality of niobium wires in electrically parallel connection.
It is a common constructional feature of the hitherto known exhaust tubeless systems that the previously mentioned `cold spot`, and thus also the melt of the metal additives is located on the wall of the discharge vessel during the operation of the lamp and in general at a spot which during sealing is covered by the vitreous enamel.
According to experience the exhaust tubeless systems make it possible to manufacture in a simple, reliable and economic manner and are therefore widely applied. However, if the materials used in the manufacture as well as the preparative and production processes are not kept under the most severe control, it may happen that the initial dispersion and stability of the electric and optical parameters and the ratio of `early failure lamps` to the lamps of average longevity increases to an undesirably large extent.
The starting point of the invention is the surmise that these undesirable phenomena are connected with the above-mentioned structural features, of the exhaust tubeless systems, i.e. with the position of the cold spot and of the melt, which in turn may be the result of two kinds of mechanism or phenomena.
One of the mechanisms originates from the direct contact between the vitreous enamel and the melt of the metal additives. It is known that the vitreous enamels applied for sealing the tubes are highly hygroscopic and (chemically) basic, hence they are extremely sensitive to humidity, to carbon dioxide or the presence of other impurities in the environment during production. It appears that the resistance of the vitreous enamel against chemical aggression by sodium is substantially reduced by the slightest degree of contamination and this reduction of resistance is much more pronounced in the case of sodium present in the melt than sodium vapour. Due to the chemical reaction between the molten sodium and the vitreous enamel, the composition of the melt is changed and the properties of the vitreous enamel are also changed, i.e. light transparency, stability, thermal expansion, etc. Naturally these factors considerably affect the properties of discharge vessels and thus of lamps.
The other mechanism is also a consequence of the common constructional features of the hitherto known exhaust tubeless systems, namely that there is a poor heat contact or heat transmission between the cold spot and the adjacent electrode, relative to systems employing exhaust tubes. In systems using exhaust tubes the temperature of the cold spot is mainly determined by the temperature of the electrodes (assuming a given geometry of construction and conditions of external heat transfer) and the temperature of the electrodes, in turn, decisively depends on the characteristics of the arc discharge such as the temperature distribution and the spread. If e.g. the work function of the electrode changes, say increases, this demands an attendant increase of the distribution and/or extent of the arc in order to attain the required ion emission required for the arc discharge, which in turn again automatically increases the ion bombardment hitting the cathode. The necessary consequence of this is a rise in the temperature of the cold spot and thus an increase in the vapour pressure of the metal additives. Due to the increase of the vapour pressure, the operating (ignition) voltage will be increased (the characteristic curve of the arc discharge is shifted) and a higher proportion of the supply voltage, (which is constant) is used up by the discharge vessel and a lower proportion for the circuitry. Therefore, although the power input increases the current demand of the arc discharge will decrease, which means that a self-weakening negative feedback process develops.
This negative feedback is present also in the known exhaust-tubeless systems but, due to the above-mentioned feeble heat contact between the cold spot and the adjacent electrode, to a much lesser extent. However, another feedback effect becomes more appreciable, i.e. the dependence of the temperature of the cold spot on the temperature of discharge-plasma. This is because in these systems the cold spot `sees` the discharge emitted energy of which directly heats the surface of the melt of the metal additives. If we now surmise again that the work function of the electrode or for any other reason the ignition voltage of the discharge vessel increases, the energy input required by, and thus correpondingly emitted output of, the plasma will also increase. Due to the heat transfer by radiation between the plasma and the surface of the melt of the metal additives the vapour pressure will be increased, whereby the ignition voltage will be further increased. It is apparent, that this process is essentially a positive feedback.
The relative `weight` of the two kinds--negative and positive--of feedback depends on the extent of the influence exerted by the temperature of the electrode and of the plasma on the temperature of the melt of the metal additives. Since the heat-contact between the cold spot and the electrode is particularly weak in exhaust-tubeless systems using a niobium wire current lead, the positive feedback process becomes predominant. It is, therefore, evident that the effect of the positive feedback is gradually to amplify any kind of instability arising in the discharge vessel.